Dwarf species

Several species of proboscideans lived on islands and experienced insular dwarfism. This occurred primarily during the Pleistocene, when some elephant populations became isolated by fluctuating sea levels, although dwarf elephants did exist earlier in the Pliocene. These elephants likely grew smaller on islands due to a lack of large or viable predator populations and limited resources. By contrast, small mammals such as rodents develop gigantism in these conditions. Dwarf proboscideans are known to have lived in Indonesia, the Channel Islands of California, and several islands of the Mediterranean.

Elephas celebensis of Sulawesi is believed to have descended from Elephas planifrons. Elephas falconeri of Malta and Sicily was only 1 m (3 ft), and had probably evolved from the straight-tusked elephant. Other descendants of the straight-tusked elephant existed in Cyprus. Dwarf elephants of uncertain descent lived in Crete, Cyclades and Dodecanese, while dwarf mammoths are known to have lived in Sardinia. The Columbian mammoth colonised the Channel Islands and evolved into the pygmy mammoth. This species reached a height of 1.2–1.8 m (4–6 ft) and weighed 200–2,000 kg (440–4,410 lb). A population of small woolly mammoths survived on Wrangel Island, now 87 miles north of the Siberian coast, as recently as 4,000 years ago. After their discovery in 1993, they were considered dwarf mammoths. This classification has been re-evaluated and since the Second International Mammoth Conference in 1999, these animals are no longer considered to be true "dwarf mammoths".

Evolution and extinct relatives

Over 161 extinct members and three major evolutionary radiations of the order Proboscidea have been recorded. The earliest proboscids, the African Eritherium and Phosphatherium of the late Paleocene, heralded the first radiation. The Eocene included anthracobunids from the Indian subcontinent and Numidotherium, Moeritherium and Barytherium from Africa. These animals were relatively small and aquatic. Later on, genera such as Phiomia and Palaeomastodon arose; the latter likely inhabited forests and open woodlands. Proboscidean diversity declined during the Oligocene. One notable species of this epoch was Eritreum melakeghebrekristosi of the Horn of Africa, which may have been an ancestor to several later species. The beginning of the Miocene saw the second diversification, with the appearance of the deinotheres and the mammutids. The former were related to Barytherium, lived in Africa and Eurasia,while the latter may have descended from Eritreum and spread to North America.

Mounted skeleton of early proboscid Moeritherium in Japan
The second radiation was represented by the emergence of the gomphotheres in the Miocene, which likely evolved from Eritreum and originated in Africa, spreading to every continent except Australia and Antarctica. Members of this group included Gomphotherium and Platybelodon. The third radiation started in the late Miocene and led to the arrival of the elephantids, which descended from, and slowly replaced, the gomphotheres. The African Primelephas gomphotheroides gave rise to Loxodonta, Mammuthus and Elephas. Loxodonta branched off earliest, around the Miocene and Pliocene boundary, while Mammuthus and Elephas diverged later during the early Pliocene. Loxodonta remained in Africa, while Mammuthus and Elephas spread to Eurasia, and the former reached North America. At the same time, the stegodontids, another proboscidean group descended from gomphotheres, spread throughout Asia, including the Indian subcontinent, China, southeast Asia and Japan. Mammutids continued to evolve into new species, such as the American mastodon.

Woolly mammoth model at the Royal BC Museum, Victoria, British Columbia
At the beginning of the Pleistocene, elephantids experienced a high rate of speciation. Loxodonta atlantica became the most common species in northern and southern Africa but was replaced by Elephas iolensis later in the Pleistocene. Only when Elephas disappeared from Africa did Loxodonta become dominant once again, this time in the form of the modern species. Elephas diversified into new species in Asia, such as E. hysudricus and E. platycephus; the latter the likely ancestor of the modern Asian elephant.Mammuthus evolved into several species, including the well-known woolly mammoth. In the Late Pleistocene, most proboscidean species vanished during the Quaternary glaciation which killed off 50% of genera weighing over 5 kg (11 lb) worldwide.

Proboscideans experienced several evolutionary trends, such as an increase in size, which led to many giant species that stood up to 4 m (13 ft) tall. As with other megaherbivores, including the extinct sauropod dinosaurs, the large size of elephants likely developed to allow them to survive on vegetation with low nutritional value. Their limbs grew longer and the feet shorter and broader. Early proboscideans developed longer mandibles and smaller craniums, while more advanced ones developed shorter mandibles, which shifted the head's centre of gravity. The skull grew larger, especially the cranium, while the neck shortened to provide better support for the skull. The increase in size led to the development and elongation of the mobile trunk to provide reach. The number of premolars, incisors and canines decreased. The cheek teeth (molars and premolars) became larger and more specialised. The upper second incisors grew into tusks, which varied in shape from straight, to curved (either upward or downward), to spiralled, depending on the species. Some proboscideans developed tusks from their lower incisors. Elephants retain certain features from their aquatic ancestry such as their middle ear anatomy and the internal testes of the males.

There has been some debate over the relationship of Mammuthus to Loxodonta or Elephas. Some DNA studies suggest Mammuthus is more closely related to the former, while others point to the latter. However, analysis of the complete mitochondrial genome profile of the woolly mammoth (sequenced in 2005) supports Mammuthus being more closely related to Elephas. Morphological evidence supports Mammuthus and Elephas as sister taxa, while comparisons of protein albumin and collagen have concluded that all three genera are equally related to each other. Some scientists believe a cloned mammoth embryo could one day be implanted in an Asian elephant's womb.

Taxonomy

Elephants belong to the family Elephantidae, the sole remaining family within the order Proboscidea. Their closest extant relatives are the sirenians (dugongs and manatees) and the hyraxes, with which they share the clade Paenungulata within the superorder Afrotheria.[8] Elephants and sirenians are further grouped in the clade Tethytheria.[9]Traditionally, two species of elephants are recognised; the African elephant (Loxodonta africana) of sub-Saharan Africa, and the Asian elephant (Elephas maximus) of Southand Southeast Asia. African elephants have larger ears, a concave back, more wrinkled skin, a sloping abdomen and two finger-like extensions at the tip of the trunk. Asian elephants have smaller ears, a convex or level back, smoother skin, a horizontal abdomen that occasionally sags in the middle and one extension at the tip of the trunk. The looped ridges on the molars are narrower in the Asian elephant while those of the African are more diamond-shaped. The Asian elephant also has dorsal bumps on its head and some patches of depigmentation on its skin. In general, African elephants are larger than their Asian cousins.

Swedish zoologist Carl Linnaeus first described the genus Elephas and an elephant from Sri Lanka (then known as Ceylon) under the binomial Elephas maximus in 1758. In 1798, Georges Cuvier classified the Indian elephant under the binomial Elephas indicus. Dutch zoologist Coenraad Jacob Temminck described the Sumatran elephant in 1847 under the binomial Elephas sumatranus. English zoologist Frederick Nutter Chasen classified all three as subspecies of the Asian elephant in 1940. Asian elephants vary geographically in their colour and amount of depigmentation. The Sri Lankan elephant (Elephas maximus maximus) inhabits Sri Lanka, the Indian elephant (E. m. indicus) is native to mainland Asia (on the Indian subcontinent and Indochina), and the Sumatran elephant (E. m. sumatranus) is found in Sumatra. 

One disputed subspecies, theBorneo elephant, lives in northern Borneo and is smaller than all the other subspecies. It has larger ears, a longer tail, and straighter tusks than the typical elephant. Sri Lankan zoologist Paules Edward Pieris Deraniyagala described it in 1950 under the trinomial Elephas maximus borneensis, taking as his type an illustration in National Geographic. It was subsequently subsumed under either E. m. indicus or E. m. sumatranus. Results of a 2003 genetic analysis indicate its ancestors separated from the mainland population about 300,000 years ago. A 2008 study found that Borneo elephants are not indigenous to the island but were brought there before 1521 by the Sultan of Sulu from Java, where elephants are now extinct.

Elephant

Elephants are large mammals of the family Elephantidae and the order Proboscidea. Traditionally, two species are recognised, the African elephant (Loxodonta africana) and the Asian elephant (Elephas maximus), although some evidence suggests that African bush elephants and African forest elephants are separate species (L. africana and L. cyclotis respectively). Elephants are scattered throughout sub-Saharan Africa, South Asia, and Southeast Asia. Elephantidae are the only surviving family of the order Proboscidea; other, now extinct, families of the order include mammoths and mastodons. Male African elephants are the largest surviving terrestrial animals and can reach a height of 4 m (13 ft) and weigh 7,000 kg (15,000 lb). All elephants have several distinctive features the most notable of which is a long trunk or proboscis, used for many purposes, particularly breathing, lifting water and grasping objects. Their incisors grow into tusks, which can serve as weapons and as tools for moving objects and digging. Elephants' large ear flaps help to control their body temperature. Their pillar-like legs can carry their great weight. African elephants have larger ears and concave backs while Asian elephants have smaller ears and convex or level backs.

Elephants are herbivorous and can be found in different habitats including savannahs, forests, deserts and marshes. They prefer to stay near water. They are considered to be keystone species due to their impact on their environments. Other animals tend to keep their distance, predators such as lions, tigers, hyenas and wild dogs usually target only the young elephants (or "calves"). Females ("cows") tend to live in family groups, which can consist of one female with her calves or several related females with offspring. The groups are led by an individual known as the matriarch, often the oldest cow. Elephants have a fission-fusion society in which multiple family groups come together to socialise. Males ("bulls") leave their family groups when they reach puberty, and may live alone or with other males. Adult bulls mostly interact with family groups when looking for a mate and enter a state of increased testosterone and aggression known as musth, which helps them gain dominance and reproductive success. Calves are the centre of attention in their family groups and rely on their mothers for as long as three years. Elephants can live up to 70 years in the wild. They communicate by touch, sight, smell and sound; elephants use infrasound, and seismic communication over long distances. Elephant intelligence has been compared with that of primates and cetaceans. They appear to have self-awareness and show empathy for dying or dead individuals of their kind.

African elephants are listed as vulnerable by the International Union for Conservation of Nature (IUCN), while the Asian elephant is classed as endangered. One of the biggest threats to elephant populations is the ivory trade, as the animals are poached for their ivory tusks. Other threats to wild elephants include habitat destruction and conflicts with local people. Elephants are used as working animals in Asia. In the past they were used in war; today, they are often put on display in zoos and circuses. Elephants are highly recognisable and have been featured in art, folklore, religion, literature and popular culture.